Method of and apparatus for making heat-sensitive stencil and heat-sensitive stencil material

ABSTRACT

When thermally perforating a thermoplastic resin film of heat-sensitive stencil material by the use of a thermal head, supply of energy to the thermal head is cut at a time the diameter of the perforations becomes not smaller than 65% and not larger than 95% of a target diameter of the perforations.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a method of and an apparatus for makinga stencil by thermally perforating a thermoplastic resin film ofheat-sensitive stencil material by a thermal head or the like, and to aheat-sensitive stencil material. More particularly, this inventionrelates to improvement in shape of perforations, printing quality andstencil making speed.

[0003] 2. Description of the Related Art

[0004] Methods of making a heat-sensitive stencil are broadly dividedinto a method in which the resin film side of the heat-sensitive stencilmaterial is brought into close contact with an original bearing thereonan image depicted in a carbon-containing material and the resin film isperforated by heat generated by the image upon exposure to infra-redrays and a method in which the resin film of the heat-sensitive stencilmaterial is imagewise perforated by two-dimensionally scanning the resinfilm side of the heat-sensitive stencil material with a device such as athermal head having an array of micro heater elements. The former methodwill be referred to as “an analog stencil making method” and the lattermethod will be referred to as “a digital stencil making method”, in thisspecification. At the present, the digital stencil making method isprevailing over the analog stencil making method since the former doesnot require carbon in the original and permits easy image processing.

[0005] When the stencil is made by the digital stencil making method, itis preferred that the perforations be discrete by pixel, and be uniformin shape and degree of penetration so that the thin lines and/or edgesof the printings show rims faithful to the original, the solid portionsof the printings has a sufficient density and the amount of ink to betransferred to each printing sheet can be well controlled not to causeoffset (the phenomenon the ink on the surface of a first printed sheetstains the back side of a second printed sheet superposed on the surfaceof the first printed sheet).

[0006] On the other hand, in order to meet the recent demand for higherimage quality, highly fine or high resolution thermal heads such as of400 dpi or 600 dpi have been in wide use as the thermal device forthermally perforating the stencil material. Such high resolution thermaldevices are generally lower than low resolution thermal devices in themaximum temperature they can provide. Accordingly, in order to perforatethe stencil material in a given size with the high resolution thermaldevice, the stencil material should be more sensitive to perforationthan when it is perforated by the low resolution thermal device.Further, since the number of perforations (pixels) increases as theresolution increases, it is preferred that the time required to formeach perforation be shortened, that is, each perforation be formed at ahigher speed. Thus, physical properties of the resin film, the structureof the thermal head, and the method of controlling the thermal head formeeting these demands have been searched for.

[0007] The thermoplastic resin film for the heat-sensitive stencilmaterial produces shrinkage stress and is perforated by shrinkage. Inorder to improve sensitivity to perforation of the heat-sensitivestencil material, there has been proposed thermoplastic resin filmhaving a specified heat shrinkage factor disclosed, for instance, inJapanese Unexamined Patent Publication No. 4(1992)-125190 orthermoplastic resin film having a specified heat shrinkage factor and aspecified heat shrinkage stress disclosed, for instance, in JapaneseUnexamined Patent Publication Nos. 7(1995)-52573 and 7(1995) -68964.However, in these patent publications, the heat shrinkage factor or theheat shrinkage stress is specified on the basis of measurement of theheat shrinkage factor or the heat shrinkage stress when the film isheated several to several tens of minutes, which is very long ascompared with the time for which the film is heated in the actualperforation. Further, the measurement is static and does not reflect theactual perforation. Further, though the heat shrinkage factor or theheat shrinkage stress measured by, for instance, TMA (thermo-mechanicalanalysis) under a macroscopic and quasi-static condition where the areato be heated is not smaller than several millimeters (mm) and thetemperature change is 10° C./min or so has been reported, the behaviorof the perforations under a microscopic and dynamic condition in theactual stencil making process where the area to be heated by the thermalhead or the like is several tens of micrometers (μm) and the temperaturechange is 1° C./μs or so has not been reported. Thus the reported heatshrinkage factor or heat shrinkage stress does not conform to the actualperforation.

[0008] Further, conventionally, discussion on the perforation in thestencil making process has been made not on the basis of behavior ofperforations in course of perforation but on the final state ofperforations. In such discussion, physical properties of the resin filmand the structure of the thermal head, and the method of controlling thethermal head are generally discussed in order to control the final sizeand shape of the perforations and the TMA data on the film is employedonly to indicate the sensitivity to perforation. Accordingly, theproperties of the film concerning to the degree to which theperforations are discrete by pixel and the shape of the perforations isstabilized are generally incompatible with the sensitivity toperforation of film and the speed at which the film is perforated. Thatis, when a film can be perforated so that the perforations are welldiscrete and uniform in shape, the film is less sensitive to theperforation and takes a long time to perforate. Naturally the oppositionis also true. Accordingly, in the actual design of a stencil makingsystem, a plurality of kinds of thermoplastic resin film are prepared,the sensitivity to perforation of each kind of film is determined byrepeating experiments or TMA measurements, and one of the kinds of filmwhich is most close to a target sensitivity is selected.

[0009] The general data on the heat shrinkage factor and heat shrinkagestress do not always conform to the evaluation of film obtained in theactual design of a stencil making system with respect to, for instance,discreteness and uniformity of shape of the perforations, thesensitivity to perforation and the perforating speed. As describedabove, this is because the TMA data and the like are obtained under amacroscopic and quasi-static condition whereas the actual perforation inthe actual stencil making process is effected under a microscopic anddynamic condition. Further, it is difficult to read from the TMA datathe performance of the film representing the perforating speed, thestability of the shape of perforations and the like except thesensitivity to perforation. Even about the sensitivity to perforation,it is difficult to estimate the difference in the sensitivity toperforation between film samples which are slightly different from eachother, for instance, in TMA curve since it is actually impossible toprepare a variety of film samples which are different from each other inone or more particular factor such as the TMA curve with the otherfactors held to be the same. Accordingly, when a suitable kind of resinfilm is to be selected, stencils must be actually made using a varietyof resin film samples, which adds to the development cost.

[0010] As described above, information obtained as a characteristicvalue in the stencil making experiments is only on the size and shape ofthe perforations at the time the perforations are completed.Accordingly, it has been very difficult to know, without experience andsense, how the physical properties of the resin film should be changedon the basis of the result of experiment in order to obtain a desirableform of perforation, which has been made difficult development of newproducts and improvement of the performance of the products.Unsatisfactory design of the performance of the resin film can result inthe case where the sensitivity to perforation and perforating speed aretoo poor to obtain a high-resolution stencil under a practical conditionthough the perforations are discrete and substantially uniform in shapeor in the case where the perforations are not discrete and not uniformin shape though the sensitivity to perforation and perforating speed aresatisfactory.

[0011] Thus, it has been impossible to develop, on the basis ofconventional data experimentally obtained, a method of and an apparatusfor making a stencil by thermally perforating a thermoplastic resin filmof heat-sensitive stencil material, and a thermoplastic resin film forheat-sensitive stencil material in which demands for uniformity in shapeof perforations, sensitivity to perforation and perforating speed areall satisfied.

SUMMARY OF THE INVENTION

[0012] In view of the foregoing observations and description, theprimary object of the present invention is to provide a method of and anapparatus for making a stencil by thermally perforating a thermoplasticresin film of heat-sensitive stencil material, and a thermoplastic resinfilm for heat-sensitive stencil material in which perforations can bediscrete and uniform in shape, and sensitivity to perforation andperforating speed are high.

[0013] In accordance with a first aspect of the present invention, thereis provided a method of making a stencil by thermally formingperforations arranged in both a main scanning direction and asub-scanning direction in a thermoplastic resin film of heat-sensitivestencil material by the use of a heat source which is heated throughsupply of energy, wherein the improvement comprises that supply ofenergy to the heat source is cut at a time the diameter of theperforations becomes not smaller than 65% and not larger than 95% of atarget diameter of the perforations.

[0014] It is preferred that the target diameters of the perforations inthe main scanning direction and the sub-scanning direction be set notsmaller than 45% and not larger than 80% of the scanning pitches in therespective directions.

[0015] In accordance with a second aspect of the present invention,there is provided a method of making a stencil by thermally formingperforations arranged in both a main scanning direction and asub-scanning direction in a thermoplastic resin film of heat-sensitivestencil material by the use of a heat source which is heated throughsupply of energy, wherein the improvement comprises that supply ofenergy to the heat source is cut at a time the area of the perforationsbecomes not smaller than 42% and not larger than 90% of a target area ofthe perforations.

[0016] It is preferred that the target area of the perforations be setnot smaller than 20% and not larger than 50% of the product of thescanning pitches in the main scanning direction and in the sub-scanningdirection.

[0017] In accordance with a third aspect of the present invention, thereis provided an apparatus for making a stencil comprising a heat sourcewhich is heated through supply of energy, a heat source control meanswhich supplies energy to the heat source and a scanning means whichscans a thermoplastic resin film of heat-sensitive stencil material withthe heat source to thermally form perforations arranged in both a mainscanning direction and a sub-scanning direction in the thermoplasticresin film, wherein the improvement comprises that the heat sourcecontrol means cuts supply of energy to the heat source at a time thediameter of the perforations becomes not smaller than 65% and not largerthan 95% of a target diameter of the perforations.

[0018] It is preferred that the heat source control means sets thetarget diameters of the perforations in the main scanning direction andthe sub-scanning direction to be not smaller than 45% and not largerthan 80% of the scanning pitches in the respective directions.

[0019] In accordance with a fourth aspect of the present invention,there is provided an apparatus for making a stencil comprising a heatsource which is heated through supply of energy, a heat source controlmeans which supplies energy to the heat source and a scanning meanswhich scans a thermoplastic resin film of heat-sensitive stencilmaterial with the heat source to thermally form perforations arranged inboth a main scanning direction and a sub-scanning direction in thethermoplastic resin film, wherein the improvement comprises that theheat source control means cuts supply of energy to the heat source at atime the area of the perforations becomes not smaller than 42% and notlarger than 90% of a target area of the perforations.

[0020] It is preferred that the heat source control means sets thetarget area of the perforations to be not smaller than 20% and notlarger than 50% of the product of the scanning pitches in the mainscanning and sub-scanning directions.

[0021] In accordance with a fifth aspect of the present invention, thereis provided a thermoplastic resin film for stencil material which isscanned by a heat source, which is heated through supply of energy, inboth a main scanning direction and a sub-scanning direction and isthermally formed with perforations arranged in the main scanning andsub-scanning directions in the thermoplastic resin film, wherein theimprovement comprises that

[0022] the heat shrinkable properties of the thermoplastic resin filmare such that the diameter of the perforations is enlarged after cuttingsupply of energy to the heat source to from 105% to 154% of that at thetime the supply of energy to the heat source is cut.

[0023] In accordance with a sixth aspect of the present invention, thereis provided a thermoplastic resin film for stencil material which isscanned by a heat source, which is heated through supply of energy, inboth a main scanning direction and a sub-scanning direction and isthermally formed with perforations arranged in the main scanning andsub-scanning directions in the thermoplastic resin film, wherein theimprovement comprises that

[0024] the heat shrinkable properties of the thermoplastic resin filmare such that the area of the perforations is enlarged after cuttingsupply of energy to the heat source to from 110% to 237% of that at thetime the supply of energy to the heat source is cut.

[0025] With reference to FIG. 5, “the diameter of the perforations” isdefined as follows. That is, in a perforation 21, the diameter of theperforation 21 in a given direction is a length 25 of an orthographicprojection of the inner periphery (a boundary defined by a dark regionof the inner slope of the rim to be described later in a reflectivebright-field image obtained though an optical microscope) of the rim 23(an annular thickened part generated by thermal perforation) of theperforation 21 onto a straight line 24 parallel to the given direction.

[0026] The “area of the perforation” is the area of the part 22 (FIG. 5)circumscribed by the inner periphery of the rim.

[0027] These inventors have found a method of evaluating perforationfrom a novel point of view. That is, we observed the phenomenon that asmall perforation was formed and enlarged with time when thethermoplastic resin film of the stencil material was brought intocontact with the heat source such as a thermal head by the use of asystem which could take an image in a microscopic field of view of theorder of μm at a high speed of μs. The result is shown in FIG. 6. InFIG. 6, the ordinate represents the diameter of the perforation and theabscissa represents the time from the time supply of energy to the heatsource is initiated. From FIG. 6, we have found that perforation occursin the following four stages.

[0028] In the first stage, the thermoplastic resin film is heated by aheater element (heat source) of a thermal head the temperature of whichis the highest at the center thereof and is lowered toward itsperiphery. The temperature of the film is the highest at a part incontact with the center of the heater element and as the distance fromthe part in contact with the center of the heater element increases, thetemperature of the film lowers. When the temperature of the film exceedsa shrinkage initiation temperature at which the film starts to shrink,shrinkage stress, which tends to reduce the distance between any twopoints on the film, is generated and accordingly, tension is produced inany point of areas which are not lower than the shrinkage initiationtemperature. The direction of the tension is substantially perpendicularto (just perpendicular to if thermal shrinkage is isotropic) isothermallines on the film. On the other hand, where the temperature of the filmis sufficiently low, no shrinkage stress is generated. Accordingly,resin of the film is moved away from the highest temperature point ofthe film as if it slides down the temperature gradient.

[0029] In the second stage, an initial small perforation is generatednear the highest temperature point of the film.

[0030] In the third stage, the outer periphery of the initial smallperforation is pulled outward by tension from outside, whereby theperforation is enlarged (growth of the perforation by shrinkage stress).The outer periphery of the perforation is pulled outward and increasesits volume taking in resin on its path, whereby the rim is formed.

[0031] In the fourth stage, the heater element is de-energized and itstemperature lowers. As the temperature of the heater element lowers, thetemperature of the film in contact with the heater element lowers, andwhen the temperature of the film becomes lower than the shrinkageinitiation temperature, no tension acts on the rim and the shape of theperforation is fixed (end of the perforation). The diameter or the areaof the perforation as measured in this stage will be referred to as thediameter or the area of the perforation “in the final state”,hereinbelow.

[0032] Thus we have found that the aforesaid incompatible requirements,that is, discreteness of the perforations, stability in shape of theperforations, sensitivity to perforation of the stencil material andhigh speed perforation, can be balanced at a high level by setting in acertain range the ratio of the size of the perforation at the timesupply of energy to the heat source is cut to the target size of theperforation out of the various parameters obtained from the perforationsize versus energizing time curve.

[0033] That is, in terms of the diameter of the perforation, theaforesaid incompatible requirements can be balanced at a high level bycutting supply of energy to the heat source at a time the diameter ofthe perforation becomes not smaller than 65% and not larger than 95% ofa target diameter of the perforation. When supply of energy to the heatsource is cut before the diameter of the perforation becomes not smallerthan 65% of the target diameter of the perforation, sensitivity toperforation deteriorates and the perforations cannot be formed at asatisfactory speed. Whereas, when supply of energy to the heat source iscut after the diameter of the perforation becomes larger than 95% of thetarget diameter of the perforation, the perforations cannot be discreteand at the same time the shape of the perforations becomes unstable.

[0034] Further, when the target diameters of the perforation in the mainscanning direction and the sub-scanning direction, that is, thediameters in the main scanning direction and the sub-scanning directionto which the perforation is expected to be enlarged after cut of theenergy supply, are set not smaller than 45% and not larger than 80% ofthe scanning pitches in the respective directions, the amount of inktransferred through the stencil obtained can be such that offset can beavoided in solid parts while necessary density is ensured, and thincharacter parts can be sufficient in width and density.

[0035] In terms of the area of the perforation, the aforesaidincompatible requirements can be balanced at a high level by cuttingsupply of energy to the heat source at a time the area of theperforation becomes not smaller than 42% and not larger than 90% of atarget diameter of the perforation. When supply of energy to the heatsource is cut before the area of the perforation becomes not smallerthan 42% of the target area of the perforation, sensitivity toperforation deteriorates and the perforations cannot be formed at asatisfactory speed. Whereas, when supply of energy to the heat source iscut after the area of the perforation becomes larger than 92% of thetarget area of the perforation, the perforations cannot be discrete andat the same time the shape of the perforations becomes unstable.

[0036] Further, when the target area of the perforation, that is, thearea to which the perforation is expected to be enlarged after cut ofthe energy supply, is set to be not smaller than 20% and not larger than50% of the product of the scanning pitches in the main scanning andsub-scanning directions, the amount of ink transferred through thestencil obtained can be such that offset can be avoided in solid partswhile necessary density is ensured, and thin character parts can besufficient in width and density.

[0037] When the heat shrinkable properties of the thermoplastic resinfilm for the stencil material are such that the diameter of theperforation is enlarged after cutting supply of energy to the heatsource to from 105% to 154% of that at the time the supply of energy tothe heat source is cut, the perforations can be discrete, the shape ofthe perforations can be stabilized, an excellent sensitivity toperforation can be ensured and a high perforating speed can be ensured.

[0038] If the diameter of the perforation is not enlarged after cuttingsupply of energy to the heat source to 105% of that at the time thesupply of energy to the heat source is cut, sensitivity to perforationdeteriorates and the perforations cannot be formed at a satisfactoryspeed. Whereas if the diameter of the perforation is enlarged aftercutting supply of energy to the heat source larger than 105% of that atthe time the supply of energy to the heat source is cut, the shape ofthe perforations is apt to fluctuate though sensitivity to perforationis improved and the perforating speed is increased.

[0039] When the heat shrinkable properties of the thermoplastic resinfilm for the stencil material are such that the area of the perforationis enlarged after cutting supply of energy to the heat source to from110% to 237% of that at the time the supply of energy to the heat sourceis cut, the perforations can be discrete, the shape of the perforationscan be stabilized, an excellent sensitivity to perforation can beensured and a high perforating speed can be ensured.

[0040] If the area of the perforation is not enlarged after cuttingsupply of energy to the heat source to 110% of that at the time thesupply of energy to the heat source is cut, sensitivity to perforationdeteriorates and the perforations cannot be formed at a satisfactoryspeed. Whereas if the area of the perforation is enlarged after cuttingsupply of energy to the heat source larger than 237% of that at the timethe supply of energy to the heat source is cut, the shape of theperforations is apt to fluctuate though sensitivity to perforation isimproved and the perforating speed is increased.

[0041] The values of the diameter and the area of the perforations arenot as measured in the thermoplastic film laminated on the poroussupport sheet (to form a heat-sensitive stencil) but as measured in thethermoplastic film by itself. This is because it is very difficult toobserve the state of perforation and to measure the diameter and/or thearea of the perforation in a state where the thermoplastic film islaminated on the porous support sheet. However, the state of perforation(the diameter and/or the area of the perforation) as measured in thethermoplastic film by itself has a high correlation with that asmeasured in the thermoplastic film laminated on the porous supportsheet. FIGS. 7 and 8 show the correlation. In FIG. 7, the ordinaterepresents the diameters of the perforations in the final state when aheat-sensitive stencil material (a thermoplastic film and a poroussupport sheet laminated together) is perforated under various conditionsand the abscissa represents the diameters of the perforations in thefinal state when the same thermoplastic film as that employed in theheat-sensitive stencil material is perforated by itself under the sameconditions. The correlation coefficient of the graph shown in FIG. 7 is0.913. In FIG. 8, the ordinate represents the areas of the perforationsin the final state when a heat-sensitive stencil material (athermoplastic film and a porous support sheet laminated together) isperforated under various conditions and the abscissa represents theareas of the perforations in the final state when the same thermoplasticfilm as that employed in the heat-sensitive stencil material isperforated by itself under the same conditions. The correlationcoefficient of the graph shown in FIG. 8 is 0.9319. Thus it will beunderstood that the state of perforation in the thermoplastic film byitself can represent the state of perforation in the heat-sensitivestencil material comprising the thermoplastic film laminated with aporous support sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042]FIG. 1 is a schematic view showing a heat-sensitive stencil makingapparatus in accordance with an embodiment of the present invention,

[0043]FIG. 2 is a view showing the relation between the temperature ofthe heater element and the square pulse applied to the heater element,

[0044]FIG. 3 is a view showing the relation between the temperature ofthe heater element and the intermittent pulse applied to the heaterelement,

[0045]FIG. 4A is a fragmentary plan view showing an important part ofthe thermal head,

[0046]FIG. 4B is a cross-sectional view taken along line Y-Y in FIG. 4A,

[0047]FIG. 4C is a cross-sectional view taken along line X-X in FIG. 4A,

[0048]FIG. 5 is a schematic view showing a perforation,

[0049]FIG. 6 is a graph showing change in diameter of the perforationduring formation thereof,

[0050]FIG. 7 is a graph showing the correlation between the diameter ofthe perforation as measured in the thermoplastic film by itself withthat as measured in the thermoplastic film laminated on the poroussupport sheet, and

[0051]FIG. 8 is a graph showing the correlation between the area of theperforation as measured in the thermoplastic film by itself with that asmeasured in the thermoplastic film laminated on the porous supportsheet.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0052] In FIG. 1, a stencil making apparatus 8 in accordance with anembodiment of the present invention comprises a thermal head 4 having anarray of a plurality of heater elements 5 (only one is visible in FIG.1), and a platen roller 3. A heat-sensitive stencil material 1 isconveyed in the direction of arrow A when the platen roller 3 is drivenby an electric motor (not shown) and passed between the platen roller 3and the thermal head 4 with the side of a thermoplastic film 1 a of thestencil material 1 facing the thermal head 4. Thus the heater elements 5of the thermal head 4 are pressed against the thermoplastic film 1 a ofthe stencil material 1 and the thermoplastic film 1 a is perforated bythe heater elements 5 energized by a head drive circuit 6. The energysupplied to the heater elements by the head drive circuit 6 iscontrolled by a controller 7. In order to increase the perforatingspeed, the heater elements 5 are divided into a plurality of blocks, andthe head drive circuit 6 drives the heater elements 5 block by block.

[0053] In this stencil making apparatus 8, power (voltage) in the formof a continuous square wave is supplied to the heater element 5 toperforate a perforation corresponding to one pixel as shown in FIG. 2.Integration of supplied power with time is supplied energy. While poweris being supplied, the temperature of the surface of the heater element5 increases and when power supply is cut, the temperature of the surfaceof the heater element 5 comes to lower. FIG. 2 is an example of changein the temperature of the surface of the heater element 5 at its centeras measured by an infrared radiation thermometer. When the heaterelement 5 is heated in the pattern shown in FIG. 2, the part of thethermoplastic resin film of the stencil material is perforated throughheat shrinkage. The heater element 5 may be supplied with power ofintermittent waveform as shown in FIG. 3. In the case where the heaterelement 5 is supplied with power of intermittent waveform, the time thelast pulse is terminated is considered to be the time supply of energyto the heater element 5 is cut. The waveform of power supplied to theheater element 5 need not be limited to a square wave having constantpower, but may be, for instance, an analog waveform.

[0054] As shown in FIGS. 4A to 4C, the thermal head 4 is of a standardstructure of a full glaze thin film type thermal head in this particularembodiment, though need not be limited to such a structure. For example,a partial glaze thin film type thermal head or a thick film type thermalhead may be employed. In FIGS. 4A to 4C, the thermal head 4 comprises aninsulating substrate 11 (e.g., of ceramic) and a glaze layer 12 formedon a metal heat radiator (not shown) in this order. Further, a pluralityof resistor strips 13, each extending in a sub-scanning direction shownby arrow Y, are formed on the glaze layer 12 arranged in a main scanningdirection shown by arrow X electrically spaced from each other byseparating belts 16. Further, a common electrode 15 a and a discreteelectrode 15 b are formed over each resistor strip 13 opposed to eachother and spaced from each other in the sub-scanning direction. When anelectric voltage is applied between the common electrode 15 a and thediscrete electrode 15 b, an electric current flows through the resistorstrip 13 between the common electrode 15 a and the discrete electrode 15b and the resistor strip 13 generates Joule heat. That is, the part ofthe resistor strip 13 between the common electrode 15 a and the discreteelectrode 15 b forms a heater element 5. The surface of the thermal head4 is covered with a protecting layer 17 and the heater element 5(resistor strip 13) is brought into contact with the thermoplastic film1 a of the stencil material 1 with the protecting layer 17 interveningtherebetween. The stencil material 1 is two-dimensionally scanned by theheater element 5 by moving the thermal head 4, having a heater elementarray extending in the main scanning direction, with respect to thestencil material 1 in the sub-scanning direction.

[0055] It is preferred that the heat shrinkable properties of thethermoplastic resin film 1 a of the stencil material 1 be such that thediameter of the perforations is enlarged after cutting supply of energyto the heater element 5 to from 105% to 154%, preferably from 109% to140%, and more preferably from 112% to 133%, of that at the time thesupply of energy to the heater element 5 is cut. In terms of the area ofthe perforation, it is preferred that the heat shrinkable properties ofthe thermoplastic resin film 1 a of the stencil material 1 be such thatthe area of the perforations is enlarged after cutting supply of energyto the heater element 5 to from 110% to 237%, preferably from 119% to196%, and more preferably from 125% to 177%, of that at the time thesupply of energy to the heater element 5 is cut. When these conditionsare met, the perforations can be discrete and stabilized in shape, thesensitivity to perforation is improved and the perforating speed can beincreased.

[0056] It is preferred that supply of energy to the heater element 5 becut at a time the diameter of the perforations becomes not smaller than65% and not larger than 95%, preferably, not smaller than 72% and notlarger than 92%, and more preferably, not smaller than75% and not largerthan 90%, of a target diameter of the perforations. In terms of area ofthe perforation, it is preferred that supply of energy to the heaterelement 5 be cut at a time the area of the perforations becomes notsmaller than 42% and not larger than 90%, preferably, not smaller than49% and not larger than 85%, and more preferably, not smaller than 56%and not larger than 81%, of a target diameter of the perforations. Thecontroller 7 controls the head drive circuit 6 so that power supply tothe heater element 5 is cut in the manner described above.

[0057] As the thermoplastic resin film 1 a of the heat-sensitive stencilmaterial 1, polyester series resins such as polyethylene terephthalate,polyolefin series resins such as polyethylene, polypropylene,polystyrene, and the like, halogenated polymers such as polyvinylidenechloride, polyvinylidene fluoride, and the like, vinyl polymer such aspolyvinyl alcohol, and polyamide series resins may be employed. Amongthose, polyester series resin is especially preferred.

[0058] “Polyester series resins” include all the polymers obtained bypolycondensation of aromatic dicarboxylic acids, aliphatic dicarboxylicacids, or alicyclic dicarboxylic acids and diols or hydroxycarboxylicacids.

[0059] As the acid component, terephthalic acid, isophthalic acid,2,6-naphthalenedicarboxylic acid, adipic acid, sebacic acid, succinicacid, 1,4-cyclohexanedicarboxylic acid, and the like may be used. One ormore of these acids may be used. Further, a part of the oxy-acid ofhydroxybenzoic acid may be copolymerized.

[0060] As the diol component, ethylene glycol, 1,4-butanediol,1,6-hexanediol, 1,4-cyclohexanedimethanol and the like are preferred.One or more of these diols may be used. Further, various combinations oflactic acids and hydroxycarboxylic acids can be employed.

[0061] As the polyester for the polyester film, polyethyleneterephthalate, copolymer of ethylene terephthalate and ethyleneisophthalate, polybutylene terephthalate, a blend of polyethyleneterephthalate and polybutylene terephthalate,polyethylene-2,6-naphthalate, polyhexamethylene terephthalate, copolymerof hexamethylene terephthalate and 1,4-cyclohexanedimethylene, L-lacticacids, D-lactic acids and the like are preferably employed.

[0062] It is preferred that the thermoplastic resin film 1 a isbiaxially oriented. The biaxially oriented thermoplastic resin film maybe oriented in any method including inflation biaxial co-orientationmethod, stenter biaxial co-orientation method and stenter biaxialsequence orientation method.

[0063] For example, the biaxially oriented thermoplastic resin film maybe obtained by preparing un-oriented film by extruding a polymer on acast drum by T-die extrusion, orienting the un-oriented film in thelongitudinal direction by a series of heated rolls, and orienting thelongitudinally oriented film in the transverse direction on a tenter orthe like as desired. In the case of biaxial sequence orientation, thefilm is generally oriented in the longitudinal direction first and thenoriented in the transverse direction. However, the film may be orientedin the transverse direction first and then oriented in the longitudinaldirection. The thickness of the un-oriented film can be controlled byadjusting the slid width of the cap, the amount of the dischargedpolymer and the rotating speed of the cast drum. The un-oriented filmcan be oriented at a desired draw ratio by adjusting the rotating speedof the heated rolls and/or the set width of the tenter. Though need notbe limited in any direction, the draw ratio is preferably 1.5× to 8×,and more preferably 3× to 8× in both the longitudinal and transversedirections. It is preferred that the orientation temperature be betweenthe glass transition temperature (Tg) of the polyester film and the coldcrystallization temperature (Tc).

[0064] Though depending upon the sensitivity requirement on the stencilmaterial, the thickness of the thermoplastic resin film is normally 0.1to 10 μm, preferably 0.1 to 5 μm, and more preferably 0.1 to 3 μm. Whenthe thermoplastic resin film is larger than 10 μm in thickness, the filmcan become difficult to perforate and when the thermoplastic resin filmis smaller than 0.1 μm in thickness, formation of the film sometimescannot be stabilized.

[0065] It is preferred that the thermoplastic resin film 1 a has one ormore melting points in the range of 150 to 240° C., and more preferablyin the range of 160 to 230° C. When the melting point is higher than240° C., high sensitivity to perforation cannot be obtained, whereaswhen the melting point is lower than 150° C., the thermal dimensionalstability of the film deteriorates and the film curls during manufactureof the stencil or during storage of the stencil, whereby printing imagequality becomes unsatisfactory.

[0066] The thermoplastic resin film is provided with adequate slipproperties by roughening the surface in order to improve workability inthe film take-up step during manufacture of the film, the coating stepduring the stencil making, the laminating step and the printing step.Inorganic particles such as of clay, mica, titanium oxide, calciumcarbonate, kaolin, talc, wet or dry silica, alumina, zirconia and thelike and organic particles such as those including, as an ingredient,acrylic acids, styrene and the like may be employed to roughen thesurface of the resin film. The amount of the particles is preferably0.05 to 10 parts by weight and more preferably 0.1 to 3 parts by weightper 100 parts by weight of resin. The mean particle size is preferably0.01 to 3 μm and more preferably 0.1 to 2 μm. A plurality of kinds ofparticles which different in kind and mean particle size.

[0067] If necessary, the thermoplastic resin film may be added withflame retarder, thermal stabilizer, antioxidant, ultraviolet absorber,antistatic agent, pigment, dye, organic lubricant such as fatty esterand wax, anti-foam agent such as polysiloxane, and the like.

[0068] As the porous support sheet, any known porous support sheet maybe employed so long as it is permeable to printing ink. For example,silk paper or paper made of synthetic fiber (as a major component)blended with natural fiber, paper made of synthetic fiber, unwovenfabric, fabric, screen gauze and the like may be employed. As thenatural fiber, Manila hemp, kozo, mitsumata, pulp and the like aregenerally employed, and as the synthetic fiber, polyester, vinylon,nylon, rayon and the like are generally employed.

[0069] The thermoplastic resin film and the porous support sheet may belaminated in any away so long as they cannot be normally separated fromeach other and the state of lamination do not interfere with formationof perforations or passage of ink through the stencil. Generally thethermoplastic resin film and the porous support sheet are bondedtogether by adhesive. However, when the support sheet is of syntheticresin, the film and the support sheet may be thermowelded. As theadhesive, vinyl acetate series adhesives, acrylic series adhesives,vinyl chloride/vinyl acetate copolymer series adhesives, polyesterseries adhesives, urethane series adhesives and the like may begenerally employed. Ultraviolet curing adhesives such as compositions ofa photopolymerization initiator with a polyester series acrylate,urethane series acrylate, epoxy series acrylate or polyol seriesacrylate may also be employed. Among those, adhesive containing thereinan urethane series acrylate as a major component is especiallypreferred. From the viewpoint of sharpness of printings, it is preferredthat the thermoplastic resin film and the porous support sheet be bondedtogether by thermowelding without using adhesive. As the thermowelding,thermocompression bonding where the film and the support sheet arepressed against each other under an elevated temperature is generallyemployed. The thermocompression bonding may be carried out in anymanner. However, it is preferred to use heated rolls in view of easinessin processing. The stencil material may be made by thermowelding aporous support sheet of unwoven fabric of thermoplastic polymer to athermoplastic resin film during manufacture thereof and orienting thethermoplastic resin film and the support sheet. This process isadvantageous in that the resin film is reinforced by the support sheetand is prevented from being broken, whereby the resin film formation isstabilized.

[0070] It is preferred that the surface of the thermoplastic resin filmbe provided with a releasing layer in order to prevent sticking uponperforation. The releasing layer may be formed by coating a releasingagent in any manner. However, it is preferred that the releasing agentbe coated by a roll coater, a gravure coater, a reverse roll coater, abar coater or the like.

[0071] As the releasing agent, known releasing agents such as thoseincluding silicone oil, silicone series resin, fluorine series resin,surface-active agent can be employed. The releasing agent may be addedwith various additives including antistatic agent, heat-resistant agent,antioxidant, organic particles, inorganic particles, pigment and thelike. Further, in order to improve dispersion in water, the releasingagent coating solution may be added with various additives such asdispersing agent, surface-active agent, preservative, anti-foam agent.From the viewpoint of running of the thermal head and/or stain of thethermal head, the thickness of the releasing layer is preferably in therange of 0.01 μm to 0.4 μm and more preferably 0.05 μm to 0.4 μm.

[0072] In order to prove the effect of the present invention, experiment(embodiments 1 to 5 of the present invention and comparative examples 1and 2) was conducted as follows.

[0073] In the experiment, each thermoplastic resin film by itself wasperforated and the shape of the perforation was evaluated. Further thesame film was bonded to a support sheet to form a heat-sensitive stencilmaterial and a stencil was made by perforating the stencil material.Then the shape of perforations in the stencil was evaluated andprintings obtained through the stencil were evaluated. Eachthermoplastic resin film by itself was perforated under the conditionshown in the following table 1 by pressing the heater element side ofthe thermal head against the film in an stencil making apparatus whichwas the sama as that shown in FIG. 1 except that it was not providedwith the platen roller 3. The experiment was conducted at the roomtemperature.

[0074] Specifically, the thermoplastic resin film by itself wasperforated in the following manner and the shape of the perforation wasevaluated in the following manner.

[0075] A fine amount of silicone oil was coated on the surface of heaterelements of the thermal head, and thermoplastic resin film was caused toadhere to the surface of the heater elements by way of the silicone oil.In order to make the silicone oil layer between the film and the heaterelements as thinner as possible, the film was pressed against theelements with a swab to be brought into closer contact with theelements. Then this system was set to an optical microscope. Ahigh-speed video camera was set to the barrel of the microscope by wayof an image intensifier. As the high speed video camera, an Ectapro HSmotion analyzer 4540 (manufactured by Kodak) was used at a rate of40,500 frames per second (frame rate≈24.7 μs). As the image intensifier,a high-brightness high-speed gate {circle over (2)} unit C6598-40(available from HAMAMATSU PHOTONICS Co.,) was used with the exposuretime set to 10 μs. The thermal head drive system was set to supply onlyone pulse to the heater elements. The high-speed video camera was set tostart taking a picture in synchronization with start of supply of thepulse to the heater elements. The optical microscope was set so that areflective bright-field image was observed through the microscope, andthe combination of the objective and the barrel lenses were selected sothat an overall image of the perforation corresponding to one heaterelement of the thermal head was taken as large as possible. Accordingly,for a thermal head of a different resolution, a different combination ofthe objective and the barrel lenses was employed.

[0076] When a pulse was applied to the heater element of the thermalhead under the conditions described above, the video camera startedtaking a picture in synchronization with start of supply of the pulse tothe heater element. Thereafter, still images of the respective frameswere taken in by a personal computer by way of a video capture. By theuse of an image analysis software, the diameter of the perforation inthe main scanning direction, the diameter of the perforation in thesub-scanning direction, the diameter of the perforation in the directionin which the diameter was maximized were obtained on the basis of acalibrated scale. As the image analysis software, an image analysispackage MacSCOPE (Mitsuya Commercial Company) was used.

[0077] With reference to FIG. 5, “the diameter of the perforation” isdefined as follows. That is, in a perforation 21, the diameter of theperforation 21 in a given direction is a length 25 of an orthographicprojection of the inner periphery (a boundary defined by a dark regionof the inner slope of the rim in a reflective bright-field imageobtained though an optical microscope) of the rim 23 (an annularthickened part generated by thermal perforation) of the perforation 21onto a straight line 24 parallel to the given direction.

[0078] The area of the perforation is obtained by the use of theaforesaid image analysis software on the basis of the aforesaid scalefrom the images taken in. The “area of the perforation” is the area ofthe part circumscribed by the inner periphery of the rim and obtained bycutting out the part by edge enhancement and binary-coding anddetermining the area of the part by image analysis.

[0079] Embodiment 1

[0080] 20 parts by weight of polyethylene terephthalate containingtherein 2 wt % of silica 1.0 μm in mean particle size, 80 parts byweight of ethylene terephthalate-ethylene isophthalate copolymer(copolymerized with 17.5 mol % of isophthalic acid) and 0.1 parts byweight of cerotic acid myristyl were fused, kneaded and extruded with abiaxial extruder and then cut into raw material of copolymer polyesterresin (copolymerized with 14 mol % of isophthalic acid; viscosityη=0.60[Pa·s], Tm=225° C.). Then the raw material was dried under vacuumfor 3 hours at 175° C. by the use of a rotary dryer. The raw materialwas extruded by an extruder 40 mm in screw diameter with the cap of theT-die held at 270° C., and was cast on a cooling drum 300 mm indiameter, whereby un-oriented sheet 13 μm thick was obtained. Then theun-oriented sheet was oriented to 3.5 times in the longitudinaldirection by a series of heated rolls at 90° C., and the longitudinallyoriented sheet was further oriented to 3.5 times in the transversedirection by a tenter transverse stretching machine at 95° C. Further,the sheet was subjected to heat treatment at 120° C. for 10 seconds inthe tenter, whereby biaxially oriented film 1.0 μm thick was prepared.

[0081] The film by itself was perforated under the conditions shown inthe following table 1.

[0082] Further the same film was laminated with paper made of polyesterfiber 4 μm in mean fiber diameter (40 wt %) blended with Manila hempfiber 15 μm in mean fiber diameter (60 wt %) by polyvinyl acetate resincoated therebetween in an amount of 0.5 g/m². The paper was 10 g/m² inweighing and 35 μm in thickness. Then silicone releasing agent wascoated on the surface of the film in an amount of 0.1 g/m², therebyobtaining a heat-sensitive stencil material.

[0083] Further, by the use of the stencil material thus obtained, astencil was made under the conditions shown in the following table 1 andprinting was done by the use of the stencil.

[0084] Embodiment 2

[0085] 10 parts by weight of polyethylene terephthalate containingtherein 2 wt % of silica 1.5 μm in mean particle size, 90 parts byweight of ethylene terephthalate-ethylene isophthalate copolymer(copolymerized with 22.3 mol % of isophthalic acid) and 0.1 parts byweight of cerotic acid myristyl were fused, kneaded and extruded with abiaxial extruder and then cut into raw material of copolymer polyesterresin (copolymerized with 20 mol % of isophthalic acid; viscosityη=0.60[Pa·s], Tm=220° C.). Then the raw material was dried under vacuumfor 3 hours at 175° C. by the use of a rotary dryer. The raw materialwas extruded by an extruder 40 mm in screw diameter with the cap of theT-die held at 270° C., and was cast on a cooling drum 300 mm indiameter, whereby un-oriented sheet 18 μm thick was obtained. Then theun-oriented sheet was oriented to 3.5 times in the longitudinaldirection by a series of heated rolls at 85° C., and the longitudinallyoriented sheet was further oriented to 3.5 times in the transversedirection by a tenter transverse stretching machine at 90° C. Further,the sheet was subjected to heat treatment at 100° C. for 10 seconds inthe tenter, whereby biaxially oriented film 1.5 μm thick was prepared.

[0086] The film by itself was perforated under the conditions shown inthe following table 1.

[0087] Further the same film was laminated with paper made of polyesterfiber 4 μm in mean fiber diameter (40 wt %) blended with Manila hempfiber 15 μm in mean fiber diameter (60 wt %) by polyvinyl acetate resincoated therebetween in an amount of 0.5 g/m². The paper was 10 g/m² inweighing and 35 μm in thickness. Then silicone releasing agent wascoated on the surface of the film in an amount of 0.1 g/m², therebyobtaining a heat-sensitive stencil material.

[0088] Further, by the use of the stencil material thus obtained, astencil was made under the conditions shown in the following table 1 andprinting was done by the use of the stencil.

[0089] Embodiment 3

[0090] 100 parts by weight of polyethylene terephthalate copolymercontaining therein 25 mol % of ethylene terephthalate unit, containingtherein 0.4 wt % of silica 1.5 μm in mean particle size, and 0.1 partsby weight of cerotic acid myristyl were fused, kneaded and extruded witha biaxial extruder and then cut into raw material of copolymer polyesterresin (viscosity η=0.62[Pa·s], Tm=197° C.). Then the raw material wasdried under vacuum for 5 hours at 150° C. by the use of a rotary dryer.The raw material was extruded by an extruder 40 mm in screw diameterwith the cap of the T-die held at 260° C., and was cast on a coolingdrum 300 mm in diameter, whereby un-oriented sheet 18 μm thick wasobtained. Then the un-oriented sheet was oriented to 3.2 times in thelongitudinal direction by a series of heated rolls at 85° C., and thelongitudinally oriented sheet was further oriented to 3.2 times in thetransverse direction by a tenter transverse stretching machine at 90° C.Further, the sheet was subjected to heat treatment at 100° C. for 10seconds in the tenter, whereby biaxially oriented film 1.7 μm thick wasprepared.

[0091] The film by itself was perforated under the conditions shown inthe following table 1.

[0092] Further the same film was laminated with paper made of polyesterfiber 4 μm in mean fiber diameter (40 wt %) blended with Manila hempfiber 15 μm in mean fiber diameter (60 wt %) by polyvinyl acetate resincoated therebetween in an amount of 0.5 g/m². The paper was 10 g/m² inweighing and 35 μm in thickness. Then silicone releasing agent wascoated on the surface of the film in an amount of 0.1 g/m², therebyobtaining a heat-sensitive stencil material.

[0093] Further, by the use of the stencil material thus obtained, astencil was made under the conditions shown in the following table 1 andprinting was done by the use of the stencil.

[0094] Embodiment 4

[0095] The same film and heat-sensitive stencil material as thoseemployed in the embodiment 3 were used.

[0096] The film by itself was perforated under the conditions shown inthe following table 1. Further, by the use of the stencil material, astencil was made under the conditions shown in the following table 1 andprinting was done by the use of the stencil.

[0097] Embodiment 5

[0098] 80 parts by weight of L-lactic acid and 20 parts by weight ofhydroxycaproic acid were introduced into a reactor and the mixture wasstirred at 145° C., 6000 Pa for 4 hours to distill water out of themixture. Then 0.05 parts by weight of tin was added and the resultantmixture was further stirred for 3 hours, whereby low polymer wasobtained. The lower polymer was subsequently added with 0.2 parts byweight of tin and 200 parts by weight of diphenyl ether and theresultant mixture was subjected to azeotropic dehydration at 148° C.,4400 Pa, and kept react for 30 hours while distilled water and solventwere separated by a water separator and only the solvent was returned tothe reactor, whereby L-lactic acid polymer solution was obtained. Thenthe solution was cooled to 40° C. and the deposit was filtered. Furtherthe deposit was washed with n-hexane and dried under vacuum. Obtainedpowder was added with 15 Kg of 0.5N hydrochloric acid and 15 Kg ofethanol and separated by filtration and dried after being stirred,whereby L-lactic acid polymer was obtained.

[0099] 100 parts by weight of the L-lactic acid polymer thus obtainedwas mixed with 0.5 parts by weight of calcium carbonate 1.5 μm in meanparticle size and the resultant mixture was extruded and pelletized by areverse biaxial extruder at 200° C. The obtained pellet was treated at50° C. under vacuum, and crystallized and dried. Then the pellet wasmelted and extruded at 200° C. by an extruder 40 mm in screw diameter,and was cast on a cooling drum 300 mm in diameter, whereby un-orientedsheet 20 μm thick was obtained. Then the un-oriented sheet was orientedto 3.5 times in the longitudinal direction by a series of heated rollsat 65° C., and the longitudinally oriented sheet was further oriented to3.5 times in the transverse direction by a tenter transverse stretchingmachine at 70° C. Further, the sheet was subjected to heat treatment at100° C. for 10 seconds in the tenter, whereby biaxially oriented film1.6 μm thick was prepared.

[0100] The film by itself was perforated under the conditions shown inthe following table 1.

[0101] Further the same film was laminated with paper made of polyesterfiber 4 μm in mean fiber diameter (40 wt %) blended with Manila hempfiber 15 μm in mean fiber diameter (60 wt %) by polyvinyl acetate resincoated therebetween in an amount of 0.5 g/m². The paper was 10 g/m² inweighing and 35 μm in thickness. Then silicone releasing agent wascoated on the surface of the film in an amount of 0.1 g/m², therebyobtaining a heat-sensitive stencil material.

[0102] Further, by the use of the stencil material thus obtained, astencil was made under the conditions shown in the following table 1 andprinting was done by the use of the stencil.

COMPARATIVE EXAMPLE 1

[0103] Polypropylene film 3.3 μm thick (Toray; Torayfan BO 2300) wasemployed.

[0104] The film by itself was perforated under the conditions shown inthe following table 1.

[0105] Further, by the use of the film, stencil material was made in thesame manner as in the embodiment 1. Then, by the use of the stencilmaterial thus obtained, a stencil was made under the conditions shown inthe following table 1 and printing was done by the use of the stencil.

COMPARATIVE EXAMPLE 2

[0106] The same polyester resin as in the embodiment 1 was cast on acooling drum by extrusion through a T-die, whereby un-oriented sheet 21μm thick was obtained. Then the un-oriented sheet was oriented to 3.5times in the longitudinal direction by a series of heated rolls at 90°C., and the longitudinally oriented sheet was further oriented to 3.5times in the transverse direction by a tenter transverse stretchingmachine at 95° C. Further, the sheet was subjected to heat treatment at120° C. for 10 seconds in the tenter, whereby biaxially oriented film1.7 μm thick was prepared. Further a heat-sensitive stencil material wasobtained in the same manner as in the embodiment 1. The film by itselfwas perforated under the conditions shown in the following table 1 andby the use of the stencil material thus obtained, a stencil was madeunder the conditions shown in the following table 1 and printing wasdone by the use of the stencil.

[0107] The result of measurement of the shapes of the perforationsformed in the thermoplastic resin film by itself in the embodiments 1 to5 and the comparative examples 1 and 2 is shown in the following tables2 and 3.

[0108] In the following table 2, the diameters in the main scanningdirection and the sub-scanning direction and the areas of theperforation at the time point t1 at which supply of energy to the heaterelement was cut and at the time t2 at which enlargement of theperforation was stopped (in the final state) are shown. Further, at thetime points t1 and t2, the shape of the perforation was measured underthe same conditions.

[0109] In the following table 3, the ratios of the diameters and theareas of the perforations at the time point t1 to those at the timepoint t2, the ratios of the diameters in the main scanning direction andthe sub-scanning direction at the time point t2 to the scanning pitchesp_(x) and p_(y) in the respective directions, and the ratio of the areaof the perforation at the time point t2 to the product of the scanningpitches in the main scanning direction and in the sub-scanning directionare shown.

[0110] The shapes of the perforations formed in the heat-sensitivestencil material in the embodiments 1 to 5 and the comparative examples1 and 2 were evaluated in the following manner and the result is shownin the following table 4.

[0111] Using the heat-sensitive stencil materials obtained in theembodiments 1 to 5 and the comparative examples 1 and 2, stencil weremade by different thermal heads (which were equal to or different fromthe thermal head employed in a stencil printer RISOGRAPH GR377 (RISOKAGAKU CORPORATION) in resolution) under the conditions shown in thetable 1. Each stencil included a black solid portion of 10 mm×10 mm (▪)and thin characters formed by one or two dots.

[0112] The perforations in the black solid portion of the stencils thusobtained were observed through an optical microscope and (1) perforatingperformance and (2) sensitivity to perforation of the heat-sensitivestencil materials were evaluated on the basis of the followingstandards.

[0113] (1) Perforating Performance of the Heat-Sensitive StencilMaterials

[0114] ⊚: Perforations were of the target size and were uniform in size.

[0115] ◯: Though perforations were substantially of the target size, theperforations somewhat fluctuated in size.

[0116] Δ: Though the size of the perforations were partly insufficient,the stencil was practically acceptable.

[0117] X: A substantial part of the perforations were unsatisfactory insize and the stencil was practically unacceptable.

[0118] (2) Sensitivity to Perforation of the Heat-Sensitive StencilMaterials

[0119] ⊚: Perforations of the target size were obtained with very smallenergy.

[0120] ◯: Perforations of the target size were obtained with relativelysmall energy.

[0121] Δ: Relatively large energy was required to obtain perforations ofthe target size.

[0122] X: Large energy was required to obtain perforations of the targetsize and perforations of the target size sometimes could not beobtained.

[0123] Using the stencils obtained in the embodiments 1 to 5 and thecomparative examples 1 and 2, printing was done and the printingsobtained were evaluated.

[0124] The stencils were manually mounted on the printing drum of astencil printer RISOGRAPH GR377 (RISO KAGAKU CORPORATION), and printingwas done at the room temperature using RISOGRAPH INK GR-HD under thestandard conditions of RISOGRAPH GR377 (power source ON). The printingsobtained were evaluated on (3) quality of the solid portion, (4) blur inthe thin characters, (5) saturation in the thin characters and (6)offset on the basis of the following standards. The result is shown inthe following table 4.

[0125] (3) Quality of the Solid Portions.

[0126] The degree of fluctuation in density by parts (microscopic partsnot larger than about 1 mm in cycle) due to fluctuation in mean densityand shape of the perforations were subjectively evaluated on the basisof the following standards.

[0127] ⊚: Density was sufficient and no fluctuation in density was felt.

[0128] ◯: Slight fluctuation in density was felt but density waspractically acceptable. Both reproducibility of solid portions in textoriginals and reproducibility of tones of picture originals wereacceptable.

[0129] Δ: Though reproducibility of solid portions in text originals wasacceptable, density was slightly poor and reproducibility of tones ofshadow portions of picture originals was insufficient.

[0130] X: Density was poor and fluctuation in density was remarkable.Both reproducibility of solid portions in text originals andreproducibility of tones of shadow portions of picture originals wereunacceptable.

[0131] (4) Blur in the Thin Characters.

[0132] The degree of blur (interruption of a pattern which was to becontinuous) in the thin characters due to fluctuation in shape of theperforations were subjectively evaluated on the basis of the followingstandards.

[0133] ⊚: No blur was felt.

[0134] ◯: Though slight blur was felt, reproducibility of thincharacters (black characters on a white ground) in text originals andreproducibility of tones of highlight portions of picture originals wereboth acceptable.

[0135] Δ: Though reproducibility of thin characters (black characters ona white ground) in text originals was acceptable, reproducibility oftones of highlight portions of picture originals was poor.

[0136] X: Blur was remarkable and reproducibility of thin characters(black characters on a white ground) in text originals andreproducibility of tones of highlight portions of picture originals wereboth unacceptable.

[0137] (5) Saturation in the Thin Characters.

[0138] The degree of saturation in the thin characters (loss of thewhite ground between adjacent two patterns) due to fluctuation in shapeof the perforations were subjectively evaluated on the basis of thefollowing standards.

[0139] ⊚: No saturation was felt.

[0140] ◯: Though slight saturation was felt, reproducibility of thincharacters (black characters on a white ground) in text originals andreproducibility of tones of shadow portions of picture originals wereboth acceptable.

[0141] Δ: Though reproducibility of thin characters (black characters ona white ground) in text originals was acceptable, reproducibility oftones of shadow portions of picture originals was poor.

[0142] X: Saturation was remarkable and reproducibility of thincharacters (black characters on a white ground) in text originals andreproducibility of tones of shadow portions of picture originals wereboth unacceptable.

[0143] (6) Offset

[0144] The degree of offset (the back side of a printed sheet is stainedby ink on the surface of the preceding printed sheet) was subjectivelyevaluated on the basis of the following standards.

[0145] ⊚: No offset was felt.

[0146] ◯: Though slight offset was felt, the offset was at a such alevel as to involve no problem even in originals where the solid portionwas large and a large amount of ink was transferred to the printings.The printings were acceptable for a formal use.

[0147] Δ: Offset was at a level such that no problem was involved inparts such as thin characters (black characters on a white ground) orhighlight portions where the amount of ink transferred to the printingswas small but stain was remarkable in the part such as a large solidportion where the amount of ink transferred to the printings was large.The printings were unacceptable for a formal use though acceptable foran informal use.

[0148] X: Offset was remarkable almost over the entire area of theoriginal. The printings were unacceptable for both a formal use and aninformal use.

[0149] As shown in the table 3, the ratios dx₁/dx₂ of the diameters ofthe perforations in the main scanning direction at the time point t1 atwhich supply of energy to the heater element was cut to those at thetime point t2 at which enlargement of the perforations was stopped (inthe final state) (and the reciprocals thereof dx₂/dx₁) in theembodiments 1 to 5 were 93% (107%), 80% (126%), 73% (138%), 69% (145%)and 91% (110%). Further, the ratios dy₁/dy₂ of the diameters of theperforations in the sub-scanning direction at the time point t1 at whichsupply of energy to the heater element was cut to those at the timepoint t2 at which enlargement of the perforations was stopped (in thefinal state) (and the reciprocals thereof dy₂/dy₁) in the embodiments 1to 5 were 95% (105%), 86% (117%), 83% (121%), 75% (133%) and 86% (117%).Further, the ratios a₁/a₂ of the areas of the perforations at the timepoint t1 at which supply of energy to the heater element was cut tothose at the time point t2 at which enlargement of the perforations wasstopped (in the final state) (and the reciprocals thereof a₂/a₁) in theembodiments 1 to 5 were 89% (113%), 68% (147%), 60% (166%), 52% (194%)and 78% (128%). As can be understood from the table 4, the evaluation ofthe perforating performance and sensitivity to perforation of thestencil materials of the embodiments 1 to 5 were all satisfactory.Further, evaluation of the printings printed by the use of the stencilsmade of the stencil materials of embodiments 1 to 5 on quality of thesolid portion, blur in the thin characters, saturation in the thincharacters and offset were all satisfactory.

[0150] To the contrast, as shown in the table 3, the ratios dx₁/dx₂ ofthe diameters of the perforations in the main scanning direction at thetime point t1 to those at the time point t2 (and the reciprocals thereofdx₂/dx₁) in the comparative examples 1 and 2 were 41% (243%) and 97%(103%). Further, the ratios dy₁/dy₂ of the diameters of the perforationsin the sub-scanning direction at the time point t1 to those at the timepoint t2 (and the reciprocals thereof dy₂/dy₁) in the comparativeexamples 1 and 2 were 45% (224%) and 99% (101%). Further, the ratiosa₁/a₂ of the areas of the perforations at the time point t1 to those atthe time point t2 (and the reciprocals thereof a₂/a₁) in the comparativeexamples 1 and 2 were 18% (546%) and 96% (104%). As can be understoodfrom the table 4, the evaluation of the perforating performance andsensitivity to perforation of the stencil materials of the comparativeexamples 1 and 2 were unsatisfactory. Further, evaluation of theprintings printed by the use of the stencils made of the stencilmaterials of the comparative examples 1 and 2 were unsatisfactory onquality of the solid portion and blur in the thin characters.

[0151] On the basis of the fact that the state of perforation asmeasured in the thermoplastic film by itself has a high correlation withthat as measured in the thermoplastic film laminated on the poroussupport sheet as described above, the result of the above experimentproves that discreteness of the perforations can be ensured, the shapeof the perforations can be stabilized and sensitivity to perforation canbe excellent when supply of energy to the heat source is cut at a timethe diameter of the perforation becomes not smaller than 65% and notlarger than 95% (preferably 72˜92%, and more preferably 75˜90%) of atarget area of the perforation, or at a time the area of the perforationbecomes not smaller than 42% and not larger than 90% (preferably 49˜85%,and more preferably 56˜81%) of a target diameter of the perforation.

[0152] When the heat shrinkable properties of the thermoplastic resinfilm for the stencil material are such that the diameter of theperforation is enlarged after cutting supply of energy to the heatsource to from 105% to 154% of that at the time the supply of energy tothe heat source is cut, the perforations can be discrete, the shape ofthe perforations can be stabilized, an excellent sensitivity toperforation can be ensured and a high perforating speed can be ensured.Further, the result of the above experiment proves that discreteness ofthe perforations can be ensured, the shape of the perforations can bestabilized and sensitivity to perforation can be excellent when the heatshrinkable properties of the thermoplastic resin film are such that thediameter of the perforations is enlarged after cutting supply of energyto the heat source to from 105% to 154% (preferably 109˜140%, and morepreferably 112˜133%) of that at the time the supply of energy to theheat source is cut, or the area of the perforations is enlarged aftercutting supply of energy to the heat source to from 110% to 237%(preferably 119˜196%, and more preferably 125˜177%) of that at the timethe supply of energy to the heat source is cut.

[0153] Further, when the target diameters of the perforation in the mainscanning direction and the sub-scanning direction, that is, thediameters in the main scanning direction and the sub-scanning directionto which the perforation is expected to be enlarged after cut of theenergy supply, are set not smaller than 45% and not larger than 80% ofthe scanning pitches in the respective directions, or when the targetarea of the perforation, that is, the area to which the perforation isexpected to be enlarged after cut of the energy supply, is set to be notsmaller than 20% and not larger than 50% of the product of the scanningpitches in the main scanning and sub-scanning directions, the amount ofink transferred through the stencil obtained can be such that offset canbe avoided in solid parts while necessary density is ensured, and thincharacter parts can be sufficient in width and density.

[0154] Though, in the embodiments 1 to 5 described above, the stencilmaterials comprises a porous support sheet and a thermoplastic resinfilm laminated with the support sheet, the stencil materials maycomprise only the thermoplastic resin film. TABLE 1 emb. 1 emb. 2 emb. 3emb. 4 emb. 5 co. ex. 1 co. ex. 2 film polymer A B C C D E A thicknessμm 1.0 1.5 1.7 1.7 1.6 3.5 1.7 thermal resolution (main) Dpi 400 800 400600 800 300 400 head resolution (sub) Dpi 400 800 400 600 800 300 400scanning pitch (main) μm 63.5 31.8 63.5 42.3 31.8 84.7 63.5 scanningpitch (sub) μm 63.5 31.8 63.5 42.3 31.8 84.7 63.5 element size (main) μm30 15 30 20 15 45 30 element size (sub) μm 40 19 40 25 19 60 40 meanpower mW 120 49 169 107 44 197 84 energizing time μs 400 350 200 240 500470 400 energy supplied μj 48.0 17.3 33.8 25.7 22.1 92.6 33.6

[0155] TABLE 2 emb. 1 emb. 2 emb. 3 emb. 4 emb. 5 co. ex. 1 co. ex. 2 t1μs 400 350 200 240 500 470 400 t2 μs 556 660 358 358 912 852 457 dx₁ μm32.4 17.5 24.0 17.6 20.0 15.0 28.5 dy₁ μm 30.1 15.0 27.3 14.4 15.0 20.027.7 a₁ μm² 816.4 219.8 548.7 212.9 251.3 251.3 661.2 dx₂ μm 34.8 22.033.0 25.6 22.0 36.5 29.3 dy₂ μm 31.7 17.5 33.0 19.2 17.5 44.8 28.0 a₂μm² 922.3 322.4 912.6 413.0 322.4 1372.4 687.1

[0156] TABLE 3 emb. 1 emb. 2 emb. 3 emb. 4 emb. 5 co. ex. 1 co. ex. 2dx₁/dx₂ % 93 80 73 69 91 41 97 dy₁/dy₂ % 95 86 83 75 86 45 99 a₁/a₂ % 8968 60 52 78 18 96 dx₂/dx₁ % 107 126 138 145 110 243 103 dy₂/dy₁ % 105117 121 133 117 224 101 a₂/a₁ % 113 147 166 194 128 546 104 dx₂/p_(x) %54.8 69.2 52.0 60.5 69.2 43.1 46.1 dy₂/p_(y) % 49.9 55.0 52.0 45.4 55.052.9 44.1 a₂/p_(x) · p_(y) % 22.9 31.9 22.6 23.1 31.9 19.1 17.0

[0157] TABLE 4 co. co. emb. 1 emb. 2 emb. 3 emb. 4 emb. 5 ex. 1 ex. 2shape; performance ◯ ⊚ ⊚ ⊚ ◯ Δ X sensitivity ⊚ ⊚ ◯ ◯ ⊚ X ⊚ printings;solid quality ◯ ⊚ ◯ ◯ ⊚ X X blur ◯ ⊚ ⊚ ⊚ ⊚ X X saturation ⊚ ◯ ⊚ ◯ ◯ ⊚ ⊚offset ◯ ⊚ ⊚ ⊚ ◯ ◯ ◯

What is claimed is:
 1. An apparatus for making a stencil comprising aheat source which is heated through a supply of energy, a heat sourcecontrol means for supplying energy to the heat source and a scanningmeans for scanning a thermoplastic resin film of heat-sensitive stencilmaterial with the heat source to thermally form perforations arranged inboth a main scanning direction and a sub-scanning direction in thethermoplastic resin film, wherein the heat source control means cuts thesupply of energy to the heat source at a time the diameter of theperforations becomes not smaller than 65% and not larger than 95% of atarget diameter of the perforations.
 2. An apparatus as defined in claim1 in which the heat source control means sets the target diameters ofthe perforations in the main scanning direction and the sub-scanningdirection to be not smaller than 45% and not larger than 80% of thescanning pitches in the respective directions.
 3. An apparatus asdefined in claim 1 in which the heat source control means sets thetarget area of the perforations to be not smaller than 20% and notlarger than 50% of the product of the scanning pitches in the mainscanning and sub-scanning directions.
 4. An apparatus for making astencil comprising a heat source which is heated through a supply ofenergy, a heat source control means for supplying energy to the heatsource and a scanning means for scanning a thermoplastic resin film ofheat-sensitive stencil material with the heat source to thermally formperforations arranged in both a main scanning direction and asub-scanning direction in the thermoplastic resin film, wherein the heatsource control means cuts supply of energy to the heat source at a timethe area of the perforations becomes not smaller than 42% and not largerthan 90% of a target area of the perforations.
 5. An apparatus asdefined in claim 4 in which the heat source control means sets thetarget area of the perforations to be not smaller than 20% and notlarger than 50% of the product of the scanning pitches in the mainscanning and sub-scanning directions.
 6. An apparatus as defined inclaim 4 in which the heat source control means sets the target diametersof the perforations in the main scanning direction and the sub-scanningdirection to be not smaller than 45% and not larger than 80% of thescanning pitches in the respective directions.